Grub2: boot USB from GRUB prompt

When the PC will not boot from USB, you can use GRUB2 to boot from the USB.

In the GRUB menu: Press 'c' to go the GRUB prompt

grub> ls
(hd0) (hd0,msdos1) ...
grub> set root=hd0,msdos1 
grub> chainloader /efi/boot/bootx64.efi
grub> boot

The USB boot menu will appear.

The above is of course an example, every situation will be different. But the idea is the same.